The present invention relates to internal shoe drum brakes for motor vehicles having a manually actuated parking brake means, and more particularly to an improvement in and relative to a wear compensating adjuster which automatically adjusts a clearance between a brake drum and each brake shoe, that is; a so-called shoe clearance, to compensate for wear of linings upon release stroke of a manually actuated parking brake means.
In internal shoe drum brakes, a manually actuated brake means comprises an extensible strut having one end engaged by one of two brake shoes, a cross shaft on which the strut is rotatably mounted, and a toggle lever securely mounted on the cross shaft for rotation therewith and having an actuating arm engaged by the other one of the brake shoes and an operating arm operatively connected to a parking brake lever mounted within a passenger compartment. When the parking brake lever is actuated to apply brakes, the toggle lever will be rotated to cause the actuating arm to press the corresponding brake shoe toward a brake drum and the reaction force is transmitted to the other brake shoe through the cross shaft and the strut to press it toward the drum, thus expanding the brake shoes toward the brake drum. A wear compensating adjuster in these brakes comprises an adjust lever with a pawl engageable with ratchet teeth on a rotary member which may be rotated to extend the strut. The adjust lever is rotatably mounted on the cross shaft with as adjust spring and a stopper pin at a predetermined angular position relative to the cross shaft. If wear compensation is required, the pawl of the adjust lever will ride over at least one ratchet tooth on the rotary member of the strut upon rotational movement of the toggle lever during actuation of the parking brake lever to apply brakes and subsequently the rotary member will be rotated to extend the strut in response to return rotational movement of the adjust lever following the return rotational movement of the toggle lever upon release stroke of the parking brake lever.
Japanese patent application laid open to public inspection for opposition purpose under No. 50-5346 (corresponding to U.S. Pat. No. 3,621,947, patented Nov. 23, 1971) disloses a wear compensating adjuster in an internal shoe drum brake in which an adjust lever with a pawl which is rotatably mounted on a cross shaft has a slot to receive one end of a torsion spring which surrounds the cross shaft and has its other end anchored to a dowel pin projecting from the cross shaft and the torsion spring has a projecting end engaging with a shoulder of the toggle lever. In this spring arrangement, since the torsion spring acts between the adjust lever and the toggle lever only to produce internal forces within the assembly of the adjust lever, the cross shaft and the toggle lever, no force will be produced to press an actuating arm of the toggle lever projecting outwardly through a back plate against the outside of the back plate to press the cross shaft against the inside of the back plate. The strut, therefore, is held in its proper position mainly by an axial load applied thereto by the shoe return springs. This causes a problem that upon brake application of a parking brake lever during brake application of a service brake, when the axial load is relieved of, the pawl rolls the strut through a ratchet rotary member as the adjust lever pivots about the cross shaft, making accurate wear compensating adjustment impossible.
Another problem is that the strut rattles to abut the back plate to make a noise during operation of the vehicle on a rough road surface having bumps with the service brake applied because the axial load on the strut is relieved of under this operating condition.
To solve the problems as above, it is possible to provide means for pressing the cross shaft against the back plate. This, however, causes a cost increase because additional components must be added. Another shortcoming is that since a space sufficiently large enough to accommodate such additional components is not available adjacent a cross shaft, it is quite difficult to install these additional compoments.
The spring arrangement as described above requires a bulky torsion spring to apply a force great enough to a pawl to rotate a ratchet rotary member.
The spring arrangement as described above will require a torsion spring bulky enough to apply sufficient torque and compression to an adjust lever for designed operation of strut extending means including a pawl and a cooperating ratchet rotary member and thus the size of an adjust lever, a cross shaft and a toggle lever must be large and strong enough to withstand stress caused by the bulky torsion spring. Therefore, with this spring arrangement, it is quite difficult to make an internal shoe drum brake compact.